数控开关电源外文翻译

power supply cookbook

Marty Brown

1．The Role of the Power Supply within the System and Design Program

The power supply assumes a very unique role within a typical system. In many respects, it is the mother of the system. It gives the system life by providing consistent and repeatable power to its circuits. It defends the system against the harsh world outside the confines of the enclosure and protects its wards by not letting them do harm to themselves. If the supply experiences a failure within itself, it must fail gracefully and not allow the failure to reach the system.

Alas, mothers are taken for granted, and their important functions are not appreciated. The power system is routinely left until late in the design program for two main reasons. First, nobody wants to touch it because everybody wants to design more exciting circuits and rarely do engineers have a background in power systems. Secondly, bench supplies provide all the necessary power during the system debugging stage and it is not until the product is at the integration stage that one says “Oops, we forgot to design the power supply!” All too frequently,the designer assigned to the power supply has very little experience in power supply design and has very little time to learn before the product is scheduled to enter production.

This type of situation can lead to the “millstone effect” which in simple terms means “You designed it, you fix it ( forever).” No wonder no one wants to touch it and, when asked, disavows any knowledge of having ever designed a power supply.

1.1 Getting Started. This Journey Starts with the First Question

In order to produce a good design, many questions must be asked prior to the beginning of the design process. The earlier they are asked the better off you are. These questions also avoid many problems later in the design program due to lack of communication and forethought. The basic questions to be asked include the following.

From the marketing department

1. From what power source must the system draw its power? There are different design approaches for each power system and one can also get information as to what adverse operating conditions are experienced for each.

2. What safety and radio frequency interference and electromagnetic interference (RFI/EMI) regulations must the system meet to be able to be sold into the target market? This would affect not only the electrical design but also the physical design.

3. What is the maintenance philosophy of the system? This dictates what sort of protection schemes and physical design would match the application.

4. What are the environmental conditions in which the product must operate? These are temperature range, ambient RF levels, dust, dirt, shock, vibration, and any other physical considerations.

5. What type of graceful degradation of product performance is desired when portions of the product fail? This would determine the type of power busing scheme and power sequencing that may be necessary within the system.

From the designers of the other areas of the product

1. What are the technologies of the integrated circuits that are being used within the design of the system? One cannot protect something, if one doesn’t know how it breaks.

2. What are the “best guess” maximum and minimum limits o f the load current and are there any intermittent characteristics in its current demand such as those presented by motors, video monitors, pulsed loads, and so forth? Always add 50 percent more to what is told to you since these estimates always turn out to be low. Also what are the maximum excursions in supply voltage that the designer feels that the circuit can withstand. This dictates the design approaches of the cross-regulation of the outputs, and feedback compensation in order to provide the needs of the loads.

3. Are there any circuits that are particularly noise-sensitive? These includeanalog-to-digital and digital-to-analog converters, video monitors, etc. This may dictate that the supply has additional filtering or may need to be synchronized to the sensitive circuit.

4. Are there any special requirements of power sequencing that are necessary for